1. Technical Field
The embodiments herein generally relates to a method of processing seismic data and particularly to a method of processing multi-component marine seismic data. The embodiments herein more particularly relates to a method of processing multi-component marine seismic data in order to estimate the shear wave properties of the seabed and the subsurface carbonate reservoirs.
2. Description of the Related Art
Seismic acquisition using a source in the water and receivers on the sea-bottom is now a standard industry operation. The normal operation is to use a seismic source in the water towed behind a boat (as shown in FIG. 1). This source will generate pressure waves (P-waves) in the water. There is then a set of recording instruments on the sea-bottom. They usually are assembled in a long cable, but single instruments (nodes) might also be used. Typically each instrument will have a 3-component geophone and a hydrophone (therefore called 4-component) or only a vertical geophone and a hydrophone (therefore called 2-component). The 3-component geophone will measure the particle movement of the sea-bottom along 3-orthogonal axis while the hydrophone will measure the pressure variation in the water at the sea-bottom. As mentioned, sometimes only a vertical component and a hydrophone are used. For the problem discussed herein, both 2- and 4-component systems are relevant.
In the existing techniques, the particle movement and the pressure variations caused by the seismic source will be recorded each time a shot is fired. The data is processed using standard, but technically advanced methods. From this a geological model of the underground is made. The most used wave-mode is the wave that has travelled as a P-wave (pressure wave) all the way from the source to a reflector where it is reflected upward and finally registered on the receivers (as shown in FIG. 1).
When deciding on the geometry of the recording system one of the most important parameters is the distance between the recording instruments (Ax, as shown in FIG. 1). The theory for this is well known. The so called Nyquist theorem states that 2 measurements are needed per wavelength. If one has wavelength in the data that are shorter than 2 times the measurement interval one will not just get bad data, one will get so called aliasing effects which in the end give positively wrong data.
Thus, if a desired shortest apparent wavelength to be measured is λ, then the distance between the instruments (Δx) must be less than or equal to λ/2. During a mapping of the underground we want to see the details that are as small as possible. This implies that we want to preserve the wavelengths as short as possible. An apparent wavelength (λa) and frequency (f) are connected by the following formulae,λa=Va/f, where Va is the apparent velocity.For the waves typically used for structural imaging (the P-waves) we require that Va might be very large, say of the order of 4000 msec for a frequency of 80 Hz. Thus one have that λa is 50 meter and an acceptable sampling interval (Δx) is 4000/(2*80)=25 m, which is the typical distance used in today's technology.
FIG. 2 illustrates a recording gathered from the North Sea (right side) and the Arabian Gulf (left side). This is the data recorded after one shot have been fired in a series of 4 shots. The wave-modes which are interested, such as the so called reflected P-waves are also indicated. These are clearly seen on the North Sea, while they are difficult to see on the Arabian Gulf data as shown in FIG. 2. This is because they are masked by a number of other waves such as Scholte waves which we cannot use for structural imaging. These Scholte waves are horizontal travelling waves trapped in the shallow upper layers because of the strong geological boundaries in the upper part of the subsurface strata. In order to remove the problematic waves properly in the subsequent processing we need to measure them correctly.
One of the existing technologies uses the sampling interval (distance between recording stations) typically of 25 meter. FIG. 3 illustrates a so called frequency-wave-number spectra for a typical data gathered in the Arabian Gulf on which a sampling interval is done. As expected, we get very strong aliasing effects in the data, which again will result in serious problems when trying to image the structures below.
Therefore, there is a need for a method of processing a multi-component marine seismic data in shallow water environment. There is also a need to define which upper frequency we want and also once the upper frequency is decided there is a need for a system with sampling interval that respects the Scholte waves and similar ground roll type waves. Further there is a need to measure these disturbing waves properly. Still there is a need to develop a system with a desired sampling interval to measure the disturbing waves properly.
The abovementioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.